This invention relates to an adaptive braking system for a fluid pressure operated vehicle braking system.
U.S. Pat. No. 3,953,083 (Latvala et al) and U.S. Pat. No. 3,958,835 (Rothfusz et al), both of which are owned by the assignee of the present invention and incorporated herein by reference, disclose an adaptive braking system which includes a skid detecting circuit which senses an incipient skidding condition of a vehicle wheel and which generates a skid detecting signal as long as the incipient skidding condition exists. The system disclosed in these patents also includes a pressure command generator which is responsive to initiation and termination of the skid detecting signal to generate a signal representing the optimum brake pressure level in the vehicle brakes, and a duty cycle generator which is responsive to the pressure command signal to actuate the vehicle brake pressure modulator which controls fluid communication to the brake actuators. Clearly, however, since the pressure level in the vehicle brake pressure actuators cannot drop below zero, the value of the pressure command signal cannot fall below a predetermined low level. However, under certain operating conditions, such as when the vehicle is operated on an extremely slippery surface such as glare ice, the skid detecting circuit may continue to generate the skid detecting signal indicating an incipient skidding condition still exists even when the value of the pressure command signal drops to its lowest possible level. When the vehicle is operated on these extremely slippery surfaces, the subsequent brake pressure build cycle which normally occurs immediately upon termination of the skid detecting signal, must be delayed, bacause the vehicle wheels are so close to a locking condition that they will almost immediately lock up before the adaptive braking system has had a chance to react to the situation to initiate another brake pressure reduction. For this reason, it is desirable under these circumstances to delay the subsequent brake pressure build cycle to be sure that the vehicle brakes do not lock. The adaptive braking system provided in the present application delays initiation of a build cycle upon termination of the skid detecting signal if the skid detecting signal is generated after the value of the signal commanded by the pressure command generator is at its lowest possible lvel. The delay is proportional to the length of time that the skid detecting signal is generated after the value of the pressure command signal has reached its lowest possible level.